HIV uses a receptor-mediated pathway in the infection of host cells. HIV-1 requires contact with two cell-surface receptors to gain entry into cells and initiate infection; CD4 is the primary receptor. CXCR4 and CCR5, members of the chemokine receptor family of proteins, serve as secondary coreceptors for HIV-1 isolates that are tropic for T-cell lines or macrophages, respectively. Deng et al. (1996) Nature 381:661-6; Doranz et al. (1996) Cell 86:1149-59; and Berger et al. (1998) Nature 391:240. CXCR4 or CCR5, in conjunction with CD4, form a functional cellular receptor for entry of certain strains of HIV into cells. Recent reports indicated that the viral envelope glycoprotein gp120 interacts directly with chemokine receptors generally at a step following CD4 binding. Lapham et al. (1996) Science 274:602-605; Moore (1997) Science 276:51; Wu et al. (1996) Nature 384:179-183; and Hesselgesser et al. (1997) Current Biology 7:112-121. Envelope variants will selectively interact with either CXCR4 or CCR5.
HIV-1 strains transmitted in vivo generally use CCR5 (CCR5 viruses). Fenyo et al. (1998) Nature 391:240; Samson et al. (1996) Nature 382:722-5; Shankarappa et al. (1999) J. Virol. 73:10489-502; and Scarlatti et al. (1997) Nature Med. 3:1259-65. These viruses typically infect macrophages and primary CD4+ lymphocytes, and do not form syncytia in vitro. Björndal et al. (1997) J. Virol. 71:7478-87. These viruses are said to be macrophage tropic (M-tropic). After primary HIV-1 infection, viral populations are usually characterized by molecular heterogeneity. Shankarappa et al. (1999); and Glushakova et al. (1999) J. Clin. Invest. 104:R7-R11.
Years after chronic infection is established, strains using CXCR4 emerge in ˜50% of infected individuals. Berger et al. (1998); Scarlatti et al. (1997); Koot et al. (1993); and Connor et al. (1997) J. Exp. Med. 185:621-8. CXCR4 strains not only infect primary T-lymphocytes but also replicate in T-cell lines and induce syncytia. Björndal et al. (1997). These viruses are said to be T-cell tropic (T-tropic). This difference in cell tropism correlates with disease progression. During HIV infection, strains isolated from individuals early in the course of their infection are usually M-tropic, while viruses isolated from approximately 50% of individuals with advanced immunodeficiency also include viruses that are T-tropic.
The finding that change from M- to T-tropic viruses over time in infected individuals correlates with disease progression suggested that the ability of the viral envelope to interact with CXCR4 represents an important feature in the pathogenesis of immunodeficiency and the development of full blown Acquired Immunodeficiency Syndrome (AIDS).
CXCR4 strains have now been shown to have a striking influence on HIV-1 disease progression. Cytopathicity toward the general CD4+ T cell population in lymphoid tissue is associated with the use of CXCR4. Glushakova et al. (1999). The emergence of CXCR4 virus is predictive of rapid depletion of CD4+ cells and acceleration of HIV-1 disease progression. Berger et al. (1998); Scarlatti et al. (1997); Koot et al. (1993); and Connor et al. (1997). A recent analysis of HIV-1 coreceptor use in infected individuals suggested that the rapid CD4+ cell decline is related to the ability of CXCR4 viruses to infect an expanded spectrum of crucial target cells as compared to CCR5 strains. Blaak et al. (2000) Proc. Natl. Acad. Sci. USA 97:1269-74. In vitro results suggest that selective blockade of CXCR4 receptors may prevent the switch from the less pathogenic CCR5 strains to the more pathogenic CXCR4 strains. Este et al. (1999) J. Virol. 73:5577-85. Coreceptor use plays a critical role in viral tropism, pathogenesis, and disease progression. Thus, a diagnosic method for use in detecting CXCR4 isolates and/or monitoring shifts in coreceptor use would be beneficial for predicting disease progression over time.
Treatment of infected individuals with highly active antiretroviral therapy (HAART) has led to a dramatic decline in both HIV-1-related illness and death. Palella et al. (1998) N. Engl. J. Med. 338:853-60. Early clinical trials demonstrated a reduction of plasma HIV-1 RNA loads to undetectable levels in the majority of treated individuals. Hammer et al. (1997) N. Engl. J. Med. 337:725-33; and Autran et al. (1997) Science 277:112-6. Subsequent studies, however, showed more limited success in achieving and maintaining viral suppression. Deeks et al. (2000) J. Inf. Dis. 181:946-53; and Mezzaroma et al. (1999) Clin. Inf. Dis. 29:1423-30. Yet many patients experienced immunologic and clinical responses to HAART without sustained suppression of plasma viremia. Deeks et al. (2000); and Mezzaroma et al. (1999).
The emergence of viral variants in connection with the failure of HAART may be associated with modified expression of the host determinants of viral tropism, including CCR5 and CXCR4. In comparison to pretherapy determinations, expression of CXCR4 was significantly increased, and CCR5 decreased, following three months of an anti-viral regimen. Giovannetti et al. (1999) Clin. Exp. Immunol. 118:87-94. Changes in coreceptor expression occurred in association with a decrease in viral load and T cell activation, and an increase in naive and memory T cells signifying peripheral redistribution of T cell compartments. In a separate study, HAART was reported to reduce the expression of CXCR4 and CCR5 in lymphoid tissue. Andersson et al. (1998) AIDS 12:F123-9. These studies did not address coreceptor usage in patients undergoing HAART. The effects of HAART on coreceptor usage by viral populations were heretofore unknown.